EPISODIC MELTWATER DISCHARGES INTO THE GULF OF MEXICO DURING THE LAST DEGLACIATION: THE BENTHIC ISOTOPE RECORD

The history of meltwater floods discharged by the Mississippi River into the Gulf of Mexico (GOM) during the last deglaciation has long attracted the attention of investigators because it provides a convenient link between the ablation of the Laurentide Ice Sheet (LIS) on land and the freshwater flooding of the ocean. Previous oxygen isotope studies of meltwater floods in sediment cores from the GOM were limited to surface-dwelling foraminifera because of the common belief that “light freshwaters float on top of denser seawater” and therefore only the planktonic foraminifera shells will record the large negative d¹⁸O excursions resulting from the episodic meltwater discharges. Here I report the history of meltwater inflows into the GOM based on continuous oxygen isotope records derived from benthic foraminifera tests that complement previously acquired planktonic-based (G. ruber and N. dutertrei) isotope records.

The oxygen isotope data are derived from four, up to 5 m long, cores taken in the northern GOM at water depths ranging from the upper bathyal (401m and 529 m) to the lower bathyal (1030 m and 1112 m). Sediment accumulation functions developed on the basis of AMS-radiocarbon dated control points and correlation of negative d¹⁸O events between cores allowed transfer from depth to time domain.

The d¹⁸O records of the benthic foraminifera U. peregrina exhibit three large negative excursions (up to 1‰) preceding the Younger Dryas cold event, and at least two negative excursions (up to 0.4‰) succeeding the Younger Dryas time. These negative isotope shifts document entrainment of ¹⁸O-depleted meltwaters in dense flows that reached the seafloor. Co-occurrence of the negative d¹⁸O shifts in the paired planktonic-benthic foraminifera records suggest that the most intense of the meltwater floods at 13.2, 12.4, 11.7, 9.4 and 9.0 ka were discharged as catastrophic hyperpycnal flows into the GOM. Paired planktonic-benthic isotope records thus provide greater insights into LIS deglacial history and links to abrupt climate changes than planktonic –based isotope records alone.